Interpretive Summary: Plant volatiles influence countless biological interactions with other organisms including human taste perception. Volatiles that positively and negatively mediate the flavor of fresh tomatoes are derived from primary nutrient precursors including amino acids, lipids and carotenoids. Specific transcription factors that regulate metabolic flux through targeted biochemical pathways provide powerful tools for manipulating and investigating these important agronomic characteristics. Working with the University of Florida (Department of Horticultural Sciences) and an international team, scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL, have discovered that the transgenic expression of a MYB transcription factor (PhODO1) in tomato fruits increases the accumulation of a specific subset of phenylpropanoid compounds. Analysis of the upregulated genes in PhODO1-expressing fruits facilitated discovery of the tomato gene encoding prephenate aminotransferase (SlPAT), which converts prephenate to arogenate. Despite the huge importance of aromatic amino acids and phenylpropanoids to the human diet, the biosynthetic pathway from prephenate to phenylalanine has not been unambiguously demonstrated. This work represents the first report of a gene encoding prephenate aminotransferase in plants and thus completes the last unknown or missing step in phenylalanine biosynthesis. This knowledge will be useful in modifying the levels of many natural products in plants including but not limited to aromatic amino acids, alkaloids, flavonoids and phenylpropanoids.

Technical Abstract:
Altering expression of transcription factors can be an effective means to coordinately modulate entire metabolic pathways in plants. It can also provide useful information concerning the identities of genes that constitute metabolic networks. Here, we used ectopic expression of a MYB transcription factor, PhODO1, to alter phenylalanine and phenylpropanoid metabolism in tomato fruits. Despite the importance of Phe synthesis has not been unambiguously determined. Microarray analysis of ripening fruits from transgenic and control plants permitted identification of a suite of co-regulated genes involved in synthesis and further metabolism of Phe. The pattern of co-regulated gene expression facilitated discovery of the tomato gene encoding prephenate aminotransferase (SIPAT), which converts prephenate to arogenate. The expression and biochemical data clearly establish an arogenate pathway involving SIPAT and arogenate dehydrates for Phe synthesis in tomato fruits. Metabolic profiling of ripe fruits further revealed large increases in the levels of a specific subset of phenylpropanoid compounds, mainly ferulic acid and conferylaldehyde as well as hexose conjugates.